THE PrERIDOPffVTA 



331 



becomes very complex and develops several concentric rings of 

 bundles, the members of which are connected with one another 

 in an intricate fashion. Masses of heavy-walled sderenchyma 

 cells are often formed in pith and cortex and aid in maintaining 

 the rigidity of the stem. 



The sporangia (Fig. 201) are borne on the back or dorsal surface 

 of the leaf in definite clusters (Fig. 199) known as fruit ing-dots 



Fig. 204. — Young sporophyte of a fern, which has developed from a fertilized 

 egg, growing out of its parent gametophyte. 



or sori (singular, sorus). Each sorus is usually covered until 

 maturity by a fold of thin, skin-like tissue, the indusium (Fig. 

 199), which arises from the leaf surface. The individual sporan- 

 gium is very small in comparison with those of bryophytes and 

 produces only a few spores. In most cases it displays around its 

 wall a characteristic ring of cells, the annidus (Fig. 201), which is 

 so constructed that upon drying it contracts like a spring, finally 



